[meteorite-list] Ibitira in thin section

From: Dave Schultz <indy1996_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:59 2005
Message-ID: <20050320011804.39396.qmail_at_web52908.mail.yahoo.com>

  I could use a few "oohs and aahs" in my life, so how
about a couple of pics!!! :)
                                      Dave


> Hello Listees, Listoids, List,
>
> The noncumulate, unbrecciated, monomict Ibitira
> eucrite with its
> mm-sized gas holes or vesicles has always been on my
> wish list
> (see O.R. Norton, p. 153 Cambridge Encyclopedia of
> Meteorites
> for a breathtaking slice measuring about 10 by 10 cm
> in size).
>
> So I was more than happy when Michael Cottingham
> offered a small
> but very characteristic slice of 0.21 grams for sale
> some time ago
> that I immediately purchased from him.
>
> Not long ago, Michael Blood offered 3 Ibitira thin
> sections, one of
> which I acquired, as this would be the ideal
> complement to my little
> slice and it even looked so similar -- almost as if
> it had been cut
> from the same piece.
>
> Today, when I looked at my Ibitira thin section
> under crossed polars and
> with several different magnifications, there were
> many "oohs" and "aahs"
> at the spectacular, colorful Ca-poor pyroxenes
> (pigeonite) most of which
> are crossed by countless parallel, fine laths of
> Ca-rich augite (so-called
> exsolution lamellae).
>
> The pyroxenes account for about 60% of my thin
> section, the second most
> abundant phase is, of course, plagioclase
> (grayish-white because it is
> chemically zoned). These plagioclase crystals show
> undulatory extinction
> when you rotate the section under your microscope
> with polars crossed.
>
> There are also several opaque inclusions throughout
> the section (they look
> black both in transmitted light and under crossed
> polars). According to the
> literature, they are ilmenite (CaTiO3), chromite
> (FeCr2O4), FeNi-metal and
> troilite.
>
> Ibitira is also said to contain the silica polymorph
> tridymite (SiO2) but I
> haven't been able to locate these laths yet as I've
> never looked for tridymite
> before. These laths can be as long as 7 mm (in NWA
> 1181) but are usually
> about 1 mm in length.
>
>
> Best Eucritic
> Wishes,
>
> Bernd
>
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Received on Sat 19 Mar 2005 08:18:04 PM PST